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Thousands of pro-democracy activists clashed with police in running scuffles in Hong Kong's gritty Mong Kok district early on Saturday in a bid to reclaim part of one of the city's largest and most volatile protest sites.

After a tense standoff lasting hours, chaos erupted as hundreds of riot police baton-charged demonstrators with shields, pepper spraying and wrestling a string of them to the ground.

The clampdown only stoked more protests, and a three-hour march by hundreds of people calling for "real full democracy" helped put the city's 28,000-strong police force further on edge.

Bands of roving protesters stalked the streets deep into the night amid a wail of sirens, sometimes pelting police with eggs, bottled water and wooden boards. Police, some bleeding, lashed out liberally with their batons to keep crowds back.

The fresh tensions came as authorities have struggled for months to find a resolution to the most serious governance crisis in the former British colony since it returned to Chinese rule in 1997.

Unrest has simmered for three straight nights since a swift and surprisingly smooth police clearance of Mong Kok's main protest encampment on Wednesday that resulted in over a hundred arrests including student leaders Joshua Wong and Lester Shum.

Amnesty International on Friday warned the police against the use of excessive force after Wong and Shum both said they were beaten during their arrests. Several reporters were also roughed up, prompting the Hong Kong Journalists Association to lodge a formal complaint and plan a Saturday protest.

"Is there a need to really use so much force to beat us," said Wong Ching-san, a young protester wearing a black jacket and flip flops. "We're not trying to cause violence but when they attack us we fight back."

Medical volunteers manning first aid stations treated scores of injured including those with head injuries, grazes and others who'd been pepper sprayed in the eyes.

A pro-democracy lawmaker who observed the clashes, Leung Yiu-chung, criticized the lack of restraint by some police.

"Some of them were deliberately inciting people," he said.

It has been two months since police first fired tear gas to disperse demonstrators from the main protest site in the Admiralty district next to government offices in the heart of the Asian financial center.

The protests, which have lasted well beyond many people's expectations, drew more than 100,000 on to the streets of Hong Kong at the peak. While numbers have dwindled, they have swelled to several thousand at weekends and at key moments given a deep-rooted frustration at China and Hong Kong's refusal to in any way offer to meet their democratic demands.

A police spokesman said on Friday officers were worried about reports of excessive force and would investigate.

Lined with jewelry and electronics shops, and grimy tenement blocks, bustling Mong Kok has been a key battleground for hardcore protesters and mobs intent on disbanding them.

The protesters, mostly students, are demanding full democracy. They have called on the city's embattled leader, Leung Chun-ying, to step down after Beijing in August ruled out free elections for Hong Kong's next leader in 2017, despite constitutional promises made by China to allow eventual universal suffrage in the city of 7.3 million.

China rules Hong Kong under a "one country, two systems" formula that accords the former British colony a degree of autonomy and freedoms not enjoyed in mainland China, with universal suffrage set as an eventual goal.